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Feb. 10, 1948. E. F. KURTZ I PROTECTIVE CIRCUIT Original Filed se v. 15, ji939 2 Sheets-Sheet 2 FIG .4.

,mau wz Rama resin, 1948 UNITED mo'mc'rlva cmcorr Edward F. Kurt s, Attleborm'Masa, assignor to Metals & Controls Corporation, Mesa, a corporation of Massachusetts Attleboro,

Original No. 2,312,915 dated March 2, 1943, ierlal No. 295,039, September 15,1939. Application for reissue July 7, 1947, Serial No. 759,414

This invention relates to protective circuits, and

with regard to certain more specific features, to-

electrical overload protectivecircuits for energy translating devices such as motors, generators,

1 transformers, electromagnets, andthe like.

Among the several objects of the invention 9 Claims. (01. 115-294) may be noted the provision of protective circuits of the class described particularly adapted for use with polyphase energy translating devices and utilizing as sensitive elements thermally responsive switches, at least two of which are provided in the circuit, which are electrically interconnected but mechanically independent; the provision of circuits of the class described in which only the indicated thermally responsive switches are required (supplementary relays, magnetic switches and the like being unnecessary); the provision of circuits of the class described wherein each of said thermally responsive switches performs a protective function governed by the current in an individual, difierent supply wire, or phase, but in which the openings of any one such switch disconnects the entire energy translation device from its power supply; the provision of circuits of the class described requiring closing of all of the separate thermally responsive switches in order to energize the energy translating device; and the provision of cir- In the accompanying drawings, in which are illustrated several of various possible embodiments of the invention,

Fig. 1 is a diagram of a circuit embodying the present invention;

Fig. 2 is a top plan view of a thermally responsive switch; I

Fig. 3 is a cross section taken substantially along line 33 of Fig. 2;

Fig. 4 is a bottom plan view of the switch of Fig. 2: and,

Figures 5, 6, and 7 are circuit diagrams illustrating alternative embodiments of the invention.

Similar reference characters indicate corresponding parts throughout the several views of the drawings.

For the sake of simplicity herein, the invention is described as it applies to a motor, but it will be obvious to anyone skilled in the art that the invention can be similarlyand equally well applied to generators, transformers; electromagnets and other forms of energy-translating devices.

There is a growing tendency to use thermaloverload protectors in conjunction with electrical motors, which control the motor directly (without intervening relays orcontactors) and are of such nature that they can be mounted directly in the motor by the motor manufacturer.

In this way, the motor manufacturer can be assured that the overload-protection switches have the proper characteristics for the particular motor; and because the protector is responsive to the actual temperature of the motor, more accurate protective results can be obtained. One of the problems in this art has been the protection of polyphase motors in a simple and eflicient manner. It has sometimes been the practice heretofore, for example, on small three-phase motors, to use two protectors each comprising a single pole switch, a thermally responsive element adapted to operate said switch, and a, heater in series with said switch and thermally adjacent to said element; one such switch being connected in each of two terminal leads. Several forms of such protectors are described in detail in John D. Bolesky applications Serial Nos. 158,107 and 189,075, filed August 9, 1937, and February 7, 1938, respectively, Serial No. 158,107 has eventuated as patent 2,199,387, dated May 7, 1940, and Serial No. 189,075 has eventuated as Patent 2,199,388, dated May 7, 1940. But this method gives unsatisfactory performance for three-phase motors although it does serve to protect the motor. For example, unless the heating of the motor at the points of mounting of the two protectors is the same, unless the currents in the two terminal leads in which the protectors are mounted are exactly the same for the two-pro-' tectors, and unless the characteristics of the two protectors are exactly the same, one will precede the other in functioning. Since absolutely uniform conditions are practically unknown in actual practice, this non-simultaneous functioning usually happens, and the motor consequently heavy stalled current will again trip this protector before the second resets and the two will into step long enough to restart the motor. This means indeterminable delay. To overcome this the manual-reset type of protector as shown in said patent application Serial No. 189,075 (Patent 2,199,388) may be used, but unless mechanical arrangements are provided so that a single push button operates both protectors, two buttons will have to be pushed each time, and, furthermore,

manual-reset protectors cost considerably more than automatic reset types, and the majority of applications are better served by the automatic type. To meet these conditions it has heretofore sometimes been the practice to interlock,

mechanically two protectors so that they operate mechanically in unison. or at least so that one may not reset while the other remains open. But such mechanical interlocking, while giving the desired performance, is more costly than the type of protectors and method of connecting herein described. Also, there are various other disadvantages in the mechanical interlocking, as such protectors must be specially .made for polyphase use, require greater skill in design and manufacture, and are naturally more complicated.

In the present invention, two thermal switches are used, each of which has a single-throw, double-pole switch actuated by a single thermal element, and the two switches are electrically connected in a circuit that accomplishes the above objects. V

While it is not a part of the present invention,

. resilient character. Switch arm I4 is mounted firmly at one end on the base by means of a rivet I5 passing therethrough, Fastened on the opposite or movable end of the switch arm I4 by welding or riveting so as to be in electrical connection therewith is an electrical contact button 16 Cooperating with, movable contact butthe construction of a switch which will satisfactorily accomplish the proper circuit operations, according to the invention, will first be described.

Referring to Figures 2, 3, and 4, numeral I' of a tab 5 formed on the disc as an integral part thereof. A supporting and adjusting screw 8 is fastened to thecenter of the disc in a rotatable manner by means of proper shoulders and abutments as indicated at numeral I, and, by engagement with a threaded hole 8 in the base I, is used to adjust the operating temperatures of the element 4. A lock nut 9 is used to hold screw G from turning after the adjustments are made.

Welded or otherwise suitably'fastened to the diametrically opposite point on the'disc from the tab 5*is a contact button I ll. Button Ill makes good electrical connection with the disc 4. Cooperating with the button III is a contact plate II which is held firmly on the base I by means of a rivet I2.

Extending through base I in a sliable manner in a holertherethrough is a push rod l3 which is actuated by the motion of the thermal element 4, and which is made of an electrical insulating material such as Bakelite." Push rod I3 in turn moves a switch arm II on the other side of the base. Switch arm I4 is made of trical conducting material having the necessary wire 20 adapted to heat the thermal element I. I

One end of heater'wire 20 is connected to' contact plate II and the other end to a terminal plate 2I, The purpose of the heater 20 is to oilset the lag intemperature between the winding 'of the motor and the thermal element 4 when the winding temperature rises rapidly from excessive current as when the motor is stalled or very heavily overloaded.

Other details of the construction of the switch need not be recited herein.- The operation of the switch is as follows:

If the disc 4 is heated either by increase in ambient temperature, or by the passage of electrical current through it and the electrical heater 20, or by a combination of these means, to a temperature suilicient toactuate it, it will snap its movable end and contact III to the right from the Fig. 3 position, for reasons indicated in the abovementioned Spencer Patent 1,448,240. In doing so, the disc I breaks the electrical connection between movable contact Ill and stationary contact II. At the same time, it also pushes to the right the insulating push rod l3, which in turn pushes the switch member II to the right, thus breaking the electrical connection between movable contactili and stationary contact I'I. Thus, it is seen that the single thermally actuated element 2 actuates what is in eflect a single-throw, double-pole switch.

Any other switch of suitable nature can be used in the circuits of the present invention.

Referring now to Fig, 1, there is shown in schematic form a circuit embodying the present invention. Numerals 22, 23,'and 24 each indicate the power leads of a polyphase source of electrical current supply. Indicated at numeral 25 is a line switch controlling the motor and representing either a manual or an automatic'switch of the type commonly used to start and stop the motor. The two thermostatically actuated protector switches of the type heretofore described are indicated at letters A and B and comprise the discs shown schematically as at to and 4b, the switch arms shown schematically as at Ida. and lib with the contacts IUa, Ila, Ilia and IIa in switch A, and Illb, IIb, lib, and I'lb in switch B.

Connections are made as follows:

Power lead 22 is connected to support plate 20., power lead 23is connected to switch arm I la through plate I50, and power lead 24 is connected to plate 2b. Contact plate "a is connected by a wire 26 to switch arm Ilb through plate Ilib; terminal plate 2Ia is connected by a wire 2'! to a terminal 28 of a motor winding 29; contact plate I1!) is connected by a wire 30 to a terminal3l of the motor winding 29; and terminal plate 2"; is

connected by a wire 32 to terminal 33 of the motor winding 29.

Considering the circuit connections for each of the motor winding terminals:

For terminal 28, current passes from the power supply through wire 22, thermostatic disc 4a,

heater 20a, and wire 21. For terminal 33. current passes through power line 24, thermostatic disc 4b, heater 20b and wire 32. "For terminal 3|, current passes through power line 23, switch arm l4a, wire 26, switch arm I41), and wire 30.

The operation of the protective circuit is as follows:

If the protectors are made to match the motor characteristics properly, and are properly mounted on or in the motor, the current traversing the heaters and discs plus the ambient temperature within the motor where the protectors are mounted will cause the discs to open when the maximum safe temperature for the motor winding is reached under any operating condition. It is not likely that the discs will function simultaneously, but either one on functioning interrupts two of the three wires 22, 23, and 24 supplying power, and hence either deenergizes the motor. Similarly, if both switches should cut out, the motor will not be re-energized until the last to reset does so, either automatically or manually depending upon the particular type of switch employed.

In Fig. 1 is shown a delta connected threephase winding 29 for the motor or device protected. However, the circuit shown serves equally well if a three-phase, Y-connected winding 34 (Fig. 6) or a two-phase, three-wire winding 35 (Fig. 7) is substituted.

The principle of this invention can be equally well applied in other types of polyphase systems such as the two-phase, four-wire system shown in Fig. 5, in which one protector switch (A and B) is connected in series with each of two windings 36 and 31, with the extra pole of each switch (A and B) in series with the other winding 31 or 36, as shown in Fig. 5, numerals 3, 39, 40', and 4| indicating the four power lines.

Also referring to Fig. 1, the circuit is likewise operative if the switch pole represented by contacts [Ba and Na were connected in series in wire 32 and/or if the pole represented by contacts i6!) and Nb were connected in series in wire 21. In fact, it is immaterial in what relationship the two switches are connected except that obviously the heaters 20a. and 20b should not be in the same line.

While a heater 20a (or 20b) is desirable for most types of motors, it is not needed for some types of energy-translating devices, particularly those in which only gradual heating can occur.

One two-pole thermostatic switch used in a circuit of the type described will serve 'to protect three-wire polyphase devices when the heater is not required. a

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As many changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. In combination with an electrical polyphase energy translating device having at least three power terminals, a protective circuit therefor including two automatically resetting double pole switches each having thermally responsive means for actuating it in accordance with the temperature of the device and current flowing in the circuit, and at least three power supply lines for the device, one of said lines being connected to one of said power terminals with one pole of one switch in series therewith, a. second line being connected to a second power terminal with one pole of the other switch'in series therewith, and

the second pole of each of said switches being connected in one of the lines other than the line in which its respective first pole is connected.

2. A circuit as set forth in claim 1, including electrical heating 'means in each of the said switches adapted to ailect their respective thermally responsive means in accordance with the electrical current passing through said heating means, the respective heating means for each switch being connected in series in one of the power lines connected through the switch it affects but not in the same power line with the other heating means. I

3. A circuit as set forth in claim 1, including. electrical heating means in each of the said switches adapted to affect their respective thermally responsive means in accordance with the electricalcurrent passing through said heating means, the respective heating means for each switch being connected in series in one of the power lines controlled by that switch.

4. In combination with an electrical polyphasev energy translating device having two windings with two power terminals each,,a protective circuit therefor including two automatically resetting double pole single throw switches each having thermally responsive means for actuating it in accordance with the temperature of the device and current flowing in the circuit, and four power supply lines for the device, one of saidpower lines being connected to one terminal of the first winding with one pole of the first switch in series therewith, a second power line being connected to the other terminal of said first winding with one pole of the second switch in series therewith, a third power line being connected electrical current passing through said heating means, the respective heating means for each switch being connected in series in one of th power lines controlled by that switch. a

. 6. In combination with an electrical polyphase energy-translating device having at least.

three power terminals, a protective circuit therefor including two automatically resetting double-pole switches each having thermally responsive means for actuating it in accordance with the temperature-of the device and current flowing in the circuit, and at least three power supply lines for the device, one of said lines being connected to one of said power terminals with one pole of at least one switchin-series therewith, a second line being connected to a second power terminal with one pole of the other switch in series therewith, and the second pole of each of said switches being connected in one of the lines other than the line in which its respective first pole is connected.

7. In combination with an electrical polyphase energy translating device having at least three power terminals. a protective circuit therefor including two automatically resetting double-pole a second switch arm, each of said switches being mounted in heat-exchange relation to the energy translating device, and at least three power lines for the device, one 01 said lines being connected to one of said power terminals with one arm or one of the switches in series therewith, a second line being connected to a second power terminal with one arm of the other switch in series therewith, a third power line being connected to a third power terminal with the remaining arms of two switches in series therewith.

8. In combination with an electrical polyp-hase energy translating device having, at least three power terminals, a protective circuit therefor including two automatically resetting double-pole switches, each of said switches including a first switch arm comprising a thermostatic element which is inherently self-heating upon passage of current therethrough and operatively coupled to a second switch arrn,-each switch also including a heater positioned to heat its thermostatic element and connected to be energized when the switch is closed, both of said switches being mounted in heat-exchange relation to the energy I translating device, and at least three power lines 8 third power terminal with the remaining arms of the switches in series therewith.

9. In combination with an electrical polyphase energy translating device having three power terminals, a protective circuit therefor including two automatically resetting double-pole switches, each switch including a. first switch arm comprising an automatically resetting, snap-acting thermostatic disc which is inherently self-heating upon passage of current therethrough. and a second switch arm coupled to the disc tor actuation thereby, each switch also including a heater positioned to heat its disc connected in series with the disc, both switches being mounted in heat exchange relation to the energy translating device, three power lines for the device, one of said lines having the said second switch arms of both of the switches in series therein. a second line having the disc and heater oi one of said switches in series therein, and the third line having the disc and heater or the other switch in series therein.

EDWARD F. KURTZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Bartmess Jan. 5, 1937 

